Polymers of C2-C20-olefins can be prepared by liquid-phase polymerization, by polymerization in the monomer (bulk polymerization), by suspension polymerization or by polymerization from the gas phase. The polymerization is usually carried out with the aid of a Ziegler-Natta catalyst which customarily comprises a titanium-containing solid component, an organic aluminum compound and an organic silane compound (EP-B 45 977, EP-A 171 200, U.S. Pat. Nos. 4,857,613, 5,288,824). Polymers of C2-C20-olefins can, however, also be obtained by polymerization with the aid of metallocene compounds or polymerization-active metal complexes. An important aspect here is that the catalyst used is metered into the polymerization reactor in an efficient manner.
The known techniques for metering finely-divided catalysts for the preparation of polyolefins have mostly been established for decades. Many of these techniques do not take account of catalyst developments which have taken place. Thus, modern high-performance catalysts require particular homogeneity of metering even in the case of small amounts. The development of metallocene catalysts has also made it necessary for fully or partially active catalysts to be introduced into the process in a safe and reliable manner.
Current and established techniques of metering catalysts are predominantly based on a portioning device which via an appropriate conveying means feeds a particular volume element into the reactor.
Examples which may be mentioned are the methods described in EP-A 0 025 137 and in U.S. Pat. No. 4,690,804, in which a dimple feeder or double-check feeder takes portions of a sedimented suspension of the catalyst from a reservoir and, by rotation through 180°, passes it to a transport stream which conveys the suspension into the reactor. The disadvantage of this method is the fixed volume of the feeder. This has the consequence that at low outputs or high catalyst productivity the number of doses per hour is very low and the process can thus easily be upset. In addition, in the case of catalysts having a high activity, there is the risk that the catalyst will not be sufficiently quickly distributed homogeneously in the reactor, which can quickly lead to lump formation when the catalyst activity is high. A further disadvantage of metering a sedimented catalyst suspension is that the catalyst concentration decreases as the fill level of the metering vessel drops and the setting of the portioning device therefore has to be adjusted continually.
A further example of a metering method is that described, inter alia, in DE-A 22 57 669. Here, the catalyst is blown into the reactor by means of nitrogen. However, this method has the disadvantage that substantial quantities of nitrogen get into the reactor and reduce the partial pressure of the monomers; they can thus have an adverse effect on the activity and the efficiency of the catalyst system.
A further possibility is to meter the catalyst into the reactor via a lock system as described in U.S. Pat. No. 3,827,830 or U.S. Pat. No. 4,123,601. However, experience has shown that such lock systems, for example systems having ball valves, are difficult to operate reliably over a prolonged period in conjunction with inorganic materials. Typical wear phenomena are, inter alia, leaks and blocked valves. This is associated with increased maintenance requirements and high costs. These metering methods, too, convey the material in portions, with the abovementioned disadvantages.
DE-A 30 26 816 describes the metering of a catalyst suspension from a stock zone into a mixing zone via a valve. Constructions of this type tend to become blocked, particularly when the valve is open for prolonged periods. Controlled metering of defined amounts is thus not possible on a long-term basis. A mixing zone as described in this application is not suitable for metering activated or partially activated catalysts since deposit formation frequently occurs.
It is an object of the present invention to remedy the disadvantages indicated and to develop a new method of metering catalysts into a reactor, by means of which the catalyst used can be introduced continuously and very homogeneously into the reactor. The metering of the catalyst should occur so that very few impurities are carried into the reactor and so that the amount of catalyst metered in is measurable. Furthermore, the method of the present invention should be able to be carried out using a metering system which is largely free of moving parts having large sealing areas, since experience has shown that pronounced wear occurs in such places and can have an adverse effect on operational reliability and operating life.
We have found that this object is achieved by a new, significantly improved method of metering catalysts into a reactor, where the catalyst is firstly suspended in a hydrocarbon in a reservoir and the suspension obtained is kept in motion by stirring and then fed via a three-way metering valve and an ejector into the actual reactor, wherein the suspension containing the catalyst is firstly discharged from the reservoir by means of a pump and continuously circulated by returning the suspension via the three-way metering valve within a closed piping system to the reservoir, subsequently setting a pressure in the reservoir which is from 0.1 to 30 bar higher than the pressure in the reactor and then continuously introducing the suspension into the reactor via a flow meter which controls the three-way metering valve and via a downstream ejector by pulse operation of the now open three-way metering valve.